The pancreas is an elongated organ which lies behind and below the stomach and consists of both exocrine and endocrine tissues. In descending order, the exocrine portion is divided into lobes, lobules, and functional secretory units known as acini. All acini eventually drain into the main pancreatic duct which joins the bile duct from the liver before it empties into the duodenum. Acinar cells comprise 80% of the pancreas and secrete enzymes in either inactive or active form which assist digestion. Epithelial cells of the ductules secrete large amounts of bicarbonate ions and water which neutralize acidic chyme as it leaves the stomach and enters the duodenum as well as the enzymes for digesting protein, carbohydrates, and fats.
The most important and abundant proteolytic enzymes are trypsin, chymotrypsin, and carboxypeptidase. The serine proteases, trypsin and chymotrypsin, split whole and partially-digested proteins into polypeptides of different sizes; then, carboxypeptidase breaks down the polypeptides into individual amino acids. Several elastases, which are also serine proteases, and nucleases, which digest nucleic acids, are also found in the pancreatic juice.
The principal enzyme for digesting carbohydrates in the gut is pancreatic amylase. It hydrolyzes starches, glycogen, and most other non-cellulosic carbohydrates to form disaccharides and trisaccharides. The main enzymes for fat digestion are pancreatic lipase, cholesterol esterase, and phospholipase. Pancreatic lipase hydrolyzes neutral fat into fatty acids and monoglycerides. Cholesterol esterase hydrolyzes cholesterol esters, and phospholipase removes fatty acid molecules from phospholipids.
The four molecules which control acinar secretion are acetylcholine and the hormones, gastrin, cholecystokinin (CCK), and secretin. Acetylcholine is released from the parasympathetic vagus and other cholinergic nerve endings, gastrin is secreted by cells of the stomach, and CCK and secretin are secreted by the upper small intestine. The gastrointestinal (GI) hormones are absorbed into the blood and transported to the pancreas where they stimulate acini to secrete enzymes and ductal cells to secrete the sodium bicarbonate and water which washes the pancreatic enzymes into the duodenum.
The endocrine pancreas consists of islets of Langerhans, whose cells are separated from the exocrine lobules and are distributed throughout the pancreas. The function of the various types of endocrine cells which make up the islets is to secrete the hormones which participate in the metabolism of proteins, carbohydrates, and fats.
The major endocrine cells are α, β, and δ cells; the minor cells are C cells, EC cells, and PP cells. About 15% of the islet cell population are α cells which are located along the periphery of islets and secrete the hormone glucagon. β cells comprise about 70% of the islet cell population, are located around the center of the islets, and secrete the hormone insulin. δ cells comprise about 10% of the population, are located close to α cells and secrete two different hormones, somatostatin and vasoactive intestinal peptide (VIP). C, EC, and PP cells make up the final 5% of the islet cell population. The function of C cells is unknown, but EC and PP cells secrete serotonin and pancreatic polypeptide, respectively.
Inflammation of the pancreas or pancreatitis may be classified as either acute or chronic by clinical criteria. With treatment, acute pancreatitis can often be cured and normal function restored. Chronic pancreatitis often results in permanent damage. The precise mechanisms which trigger acute inflammation are not understood. However, some causes in the order of their importance are alcohol ingestion, biliary tract disease, post-operative trauma, and hereditary pancreatitis. One theory provides that autodigestion, the premature activation of proteolytic enzymes in the pancreas rather than in the duodenum, causes acute pancreatitis. Any number of other factors including endotoxins, exotoxins, viral infections, ischemia, anoxia, and direct trauma may activate the proenzymes. In addition any internal or external blockage of pancreatic ducts can also cause an accumulation of pancreatic juices in the pancreas resulting cellular damage.
As is the case in inflammation of other tissues, leukocytes including monocytes, macrophages, basophils, and eosinophils infiltrate the inflamed area of the pancreas. Their primary role is to clean up the site of the inflammation; however, macrophages may produce powerful oxidants and proteases which contribute to tissue destruction. Leukocytes also secrete a range of cytokines which recruit other cells to the area.
The investigation of the critical, regulatory processes by which white cells proceed to their appropriate destination and interact with other cells is underway. The current model of leukocyte movement or trafficking from the blood to injured or inflamed tissues comprises the following steps. The first step is the rolling adhesion of the leukocyte along the endothelial cells of the blood vessel wall. This movement is mediated by transient interactions between selectins and their ligands. A second step involves cell activation which promotes a more stable leukocyte-endothelial cell interaction mediated by the integrins and their ligands. This stronger, more stable adhesion precipitates the final steps or leukocyte diapedesis and extravasation into the tissues.
The chemokine family of polypeptide cytokines possesses the cellular specificity required to explain leukocyte trafficking in different abnormal, inflammatory or diseased situations. First, chemokines mediate the expression of particular adhesion molecules on endothelial cells; and second, they generate gradients of chemoattractant factors which activate specific cell types. In addition, the chemokines stimulate the proliferation of specific cell types and regulate the activation of cells which bear specific receptors. These activities demonstrate a high degree of target cell specificity.
The chemokines are small polypeptides, generally about 70–100 amino acids (aa) in length, 8–11 kD in molecular weight and active over a 1–100 ng/ml concentration range. Initially, they were isolated and purified from inflamed tissues and characterized relative to their bioactivity. More recently, chemokines have been discovered through molecular cloning techniques and characterized by structural as well as functional analysis.
The chemokines are related through a four-cysteine motif which is based primarily on the spacing of the first two cysteine residues in the mature molecule. Currently the chemokines are assigned to one of two families, the C—C chemokines (α) and the C—X—C chemokines (β). Although exceptions exist, the C—X—C chemokines generally activate neutrophils and fibroblasts while the C—C chemokines act on a more diverse group of target cells which include monocytes/macrophages, basophils, eosinophils, T lymphocytes and others. The known chemokines of both families are synthesized by many diverse cell types as reviewed in Thomson A. (1994) The Cytokine Handbook, 2d Ed. Academic Press, NY. The two groups of chemokines will be described in turn.
At this time, relatively few C—C chemokines have been described, and they appear to have less N-terminal processing than the C—X—C chemokines. A brief description of the known human (and/or murine) C—C chemokines follows. The macrophage inflammatory proteins alpha and beta (MIP-1α and β) were first purified from stimulated mouse macrophage cell line and elicited an inflammatory response when injected into normal tissues. At least three distinct and non-allelic genes encode human MIP-1α, and seven distinct genes encode MIP-1β.
MIP-1α and MIP-1β consist of 68–69 amino acid which are about 70% identical in their acidic, mature secreted forms. They are both expressed in stimulated T cells, B cells and monocytes in response to mitogens, anti-CD3 and endotoxin, and both polypeptides bind heparin. While both molecules stimulate monocytes, MIP-1α chemoattracts the CD-8 subset of T lymphocytes and eosinophils, while MIP-1α chemoattracts the CD-4 subset of T lymphocytes. In mouse, these proteins are known to stimulate myelopoiesis.
I-309 was cloned from a human γ-δ T cell line and shows 42% amino acid identity to T cell activation gene 3 (TCA3) cloned from mouse. There is considerable nucleotide homology between the 5′ flanking regions of these two proteins, and they share an extra pair of cysteine residues not found in other chemokines. Such similarities suggest I-309 and TCA3 are species homologs which have diverged over time in both sequence and function.
RANTES is another C—C chemokine which is expressed in T cells (but not B cells), in platelets, in some tumor cell lines, and in stimulated rheumatoid synovial fibroblasts. In the latter, it is regulated by interleukins-1 and -4, transforming nerve factor and interferon-γ. The cDNA cloned from T cells encodes a basic 8 kD protein which lacks N-linked glycosylation and is able to affect lymphocytes, monocytes, basophils and eosinophils. The expression of RANTES mRNA is substantially reduced following T cell stimulation.
Monocyte chemotactic protein (MCP-1) is a 76 amino acid protein which appears to be expressed in almost all cells and tissues upon stimulation by a variety of agents. The targets of MCP-1, however, are limited to monocytes and basophils in which it induces a MCP-1 receptor:G protein-linked calcium flux (Charo I, personal communication). Two other related proteins (MCP-2 and MCP-3) were purified from a human osteosarcoma cell line. MCP-2 and MCP-3 have 62% and 73% aa identity, respectively, with MCP-1 and share its chemoattractant specificity for monocytes.
Current techniques for diagnosis of abnormalities in the inflamed or diseased tissues mainly rely on observation of clinical symptoms or serological analyses of body tissues or fluids for hormones, polypeptides or various metabolites. Patients often manifest no clinical symptoms at early stages of disease or tumor development. Furthermore, serological analyses do not always differentiate between invasive diseases and genetic syndromes which have overlapping or very similar ranges. Thus, development of new diagnostic techniques comprising small molecules such as the expressed chemokines are important to provide for early and accurate diagnoses, to give a better understanding of molecular pathogenesis, and to use in the development of effective therapies.
The pancreas is reviewed in Guyton A C (1991) Textbook of Medical Physiology, W B Saunders Co, Philadelphia; and The Merck Manual of Diagnosis and Therapy, (1992) Merck Research Laboratories, Rahway, N.J. The chemokine molecules are reviewed in Schall T J (1994) Chemotactic Cytokines: Targets for Therapeutic Development. International Business Communications, Southborough, Mass., pp 180–270; and in Paul W E (1993) Fundamental Immunology, Raven Press, New York City (NYC), pp 822–826.